There are many applications, such as navigation, astronomy, target tracking, image enhancement, where it is desirable to calculate the speed of an object or of an image of an object. Traditionally, many ways of detecting the velocity of an object have involved emitting signals that are reflected off the object, such as are involved in radar and sonar velocity detection systems. For many uses, detecting velocity by emitting such signals is satisfactory, but in other applications it presents problems. For example, in astronomy and navigation the celestial objects whose relative velocities are being measured are often so far away that it is impractical to emit and reflect signals off them. In certain reconnaissance and target tracking applications the emission of such signals would be undesirable because it would tend to alert an enemy to the location from which such signals were being emitted. And the cost, size and energy consumption of the equipment required to emit and receive such signals are often disadvantageous.
In addition to determining the velocity of objects, it is often desirable to be able to determine the velocity of images which are being photographed, televised, or otherwise recorded or transduced, so that such velocity can be compensated or accounted for. For example, time delay and integrate (TDI) imaging charge-coupled device (CCD) arrays, such as those used for airborne reconnaissance, typically require a determination of image velocity to function properly. TDI imaging CCD arrays are commonly constructed out of a plurality of closely spaced, parallel TDI CCD shift registers built on a photoelectric semiconductive substrate. Such devices have been designed to have an optical image scanned across the surface of their substrate along the length of their TDI shift registers and to have those shift registers clocked in synchronism with the motion of the optical image. This is done so that electrons which are freed under one of the TDI shift registers by the photoelectric effect of light in a given portion of the optical image are dumped into a charge packet that will be moved along by the TDI shift register in conjunction with that given portion of the optical image. When a charge packet reaches the end of its associated TDI shift register, it is fed, in parallel with all of the other charge packets which have reached the end of their associated TDI shift registers at the same time, into an output CCD shift register. The output shift register then rapidly shifts out all of the charge packets fed to it, so as to provide a series of charge packets, or pixel values, the variable charge levels of which correspond to the variable light intensity of a picture line taken from the two-dimensional optical image scanned across the TDI imaging CCD array.
In order for a TDI imaging CCD array to function properly, it is necessary that its charge packets be shifted down its shift registers at the same velocity as the image being scanned across its surface. Thus it is necessary to determine the image velocity in order to properly time the clocking circuitry that controls the rate at which the charge packets are shifted. It is also desirable to determine if the image has any velocity component in a direction perpendicular to such shift registers. Unless compensated for, such sideways velocity smears the image produced by the TDI imaging CCD because it causes charge packets and their associated portions of the image to become separated. However, once such sideways velocity has been determined it is possible to compensate for it by such means as rotating the TDI imaging CCD so that the image motion is parallel with its shift registers.